Fiber optic communication systems require high performance light source capable of generating single-mode, narrow spectral linewidth emission in the 1.3-1.56 μm wavelength range. Some of the existing semiconductor lasers, for example, InGaAsP DFB lasers can meet requirements for the light sources for optical communication systems, but fail to satisfy requirements for stable single-mode operation which is insensitive to ambient temperature change (uncooled operation) and insensitive to external optical feedback (isolator-less operation), as well as meeting the requirements of high single-mode yield and high output power.
Conventional index-coupled DFB lasers employing an index corrugation have an inherent problem in existence of two longitudinal modes with an equal threshold gain which results in poor single mode operation as shown in the article by Kogelnik and C. V. Shank, “Coupled-mode theory of distributed feedback lasers”, Journal of Applied Physics, vol. 43, no. 5, pp. 2327-2335, May 1972. One known approach to solve this problem is the use of asymmetric facet coating to each facet of the laser. The yield, however, is relatively low due to the random variations of facet grating phase introduced by cleaving.
Another approach is the incorporation of a quarter-wave phase shift, described, for example, in the article by K. Utaka, S. Akiba, K. Sakai, and Y. Matushima, “λ/4-shifted InGaAsP/InP DFB lasers”, IEEE Journal of Quantum Electronics, vol. QE-22, no.7, pp. 1042-1052, July 1986. Although this type of DFB laser with perfect AR coatings has, in principle, 100% yield, the yield deterioration due to reflectivities of a few percent is rapid. Furthermore, the output power from the front facet is relatively low due to a symmetric nature in which the same amount of power is emitted and wasted from the back facet. More specifically, so called spatial hole-burning, the reduction of carrier density caused by a too high photon density in the center of a laser cavity deteriorates the side-mode suppression and output power at high injection current levels.
An alternative approach to the mode degeneracy problem is the introduction of gain coupling. It was predicted by H. Kogelnik and C. V. Shank (see the reference cited) that the pure gain coupling provides a single-mode oscillation exactly at the Bragg wavelength. It was also predicted that an addition of even small gain coupling to index coupling (this case is called as “complex coupling”) can break the mode degeneracy, as shown in the article by E. Kapon, A. Hardy, and A. Katzir, “The effect of complex coupling coefficients on distributed feedback lasers”, IEEE Journal of Quantum Electronics, vol. QE-18, pp. 66-71, January 1982. The pure gain-coupled DFB laser has been demonstrated, for example, in the publication by Y. Luo, Y. Nakano, K. Tada et al., “Purely gain-coupled distributed feedback semiconductor lasers”, Appl. Phys. Lett., vol. 56, no. 17, pp. 1620-1622, April 1990.
It has been demonstrated that complex-coupled DFB lasers have many advantages over conventional index-coupled DFB lasers, such as high single-mode yield, less sensitivity to external optical feedback, high modulation bandwidth and reduced wavelength chirp. Among other complex-coupled DFB lasers, multi-quantum-well (MQW) DFB lasers with etched quantum wells appear to provide the highest single-mode stability up to date. For example, G. P. Li, T. Makino et al. “1.55 μm index/gain coupled DFB lasers with strained layer multiquantum-well active grating”, Electronics Letters, vol. 28, no. 18, pp. 1726-1727, August 1992, describes a complex-coupled DFB laser, having MQW active grating. The grating is patterned by etching grooves through substantially all MQW layers of the active region and regrowing InP material in the etched grooves. The high corrugation section with a larger number of quantum wells (QW's) has a larger modal gain as well as a lager real modal-index than the low corrugation section in the groove (this is called ‘in-phase” complex coupling since index coupling and gain coupling are in phase).
Both theory and experiment have confirmed that in-phase complex-coupled DFB lasers will predominantly lase on the longer wavelength side of the Bragg stop band (this lasing mode is referred to as the long Bragg mode hereafter). This can be explained by standing-wave effect: The standing wave of the long Bragg mode is mostly confined in the high corrugation section, while the standing wave of the mode at the shorter wavelength side of the Bragg stop band (this lasing mode is referred to as the short Bragg mode hereafter) is mostly confined in the low corrugation section, due to the difference of the real modal index in the two sections. Therefore, the long Bragg mode has lower threshold gain than the short Bragg mode. The mode selection is based on the standing-wave effect, not on the phase-shift effect, which results in higher single-mode yield as well as higher single-mode stability, compared to λ/4-shifted DFB lasers.
However, this type of laser has relatively low external quantum efficiency due to too large index coupling induced by periodically etched QW's, losses in the groove regions where there is little gain medium. To improve high power performance, a complex-coupled DFB laser which is similar to that described above but has several QW's not etched in the groove regions, was demonstrated in the article by H. Lu, C. Blaauw, B. Benyon, G. P. Li, and T. Makino, “High-power and high-speed performance of 1.3 μm strained MQW gain-coupled DFB lasers”, IEEE Journal of Selected Topics in Quantum Electronics, vol. 1, no. 2, pp. 375-381, June 1995. This type of DFB laser exhibits higher external quantum efficiency because the low corrugation region has several QW's which can reduce the index coupling as well as reducing the waveguide loss.
In the complex-coupled DFB lasers with periodically etched MQW's, described above, however, there are still fundamental problems such as (1) variations of the complex coupling coefficient due to variations of grating etching depth, and (2) laser performance variations due to random variations of facet grating phase. In the prior arts, the etching to make corrugations is stopped in an InGaAsP barrier layer, and an InP material is regrown in the etched grooves. The InGaAsP barrier layer has a real refractive index larger than that of the regrown material InP. The variation of the groove depth causes the variation of the thickness of the etched barrier layer InGaAsP. This results in a variation in both the modal index and the optical confinement factor of the etched groove region, which in turn a variation in the complex (index and gain) coupling coefficient. In the prior arts, in order to get a higher output power from the laser front facet, the combination of one facet AR coated and the other facet cleaved or high reflection (HR) coated are used. The reflection at the cleaved or HR coated facet causes a random variation in the facet grating phase. It was shown that even strong gain coupling cannot eliminate the variation of laser performance due to random grating phase at cleaved facet or HR coated facet, in the article by J. Hong, K. Leong, T. Makino, X. Li, and W. P. Huang, “Impact of random facet phase on modal properties of partly gain-coupled DFB lasers”, Journal of Selected Topics in Quantum Electronics, vol. 3, no. 2, pp. 555-568. April 1997.